An ion source is a device that ionizes gas molecules and then focuses, accelerates, and emits them as a narrow beam. This beam is then used for various technical and technological purposes such as cleaning, activation, polishing, thin-film coating, or etching.
For better understanding the principle of the present invention, it would be expedient to describe in detail a known ion-beam source of the type to which the invention pertains. Such an ion source is described, e.g., in Russian Patent No. 2,030,807 issued in 1995 to M. Parfenyonok, et al. The patent describes an ion source that comprises a magnetoconductive housing used as a cathode having an ion-emitting slit, an anode arranged in the housing symmetrically with respect to the emitting slit, a magnetomotance source, a working gas supply system, and a source of electric power supply.
FIGS. 1 and 2 schematically illustrate the aforementioned known ion source with a circular ion-beam emitting slit. More specifically, FIG. 1 is a sectional side view of an on-beam source with a circular ion-beam emitting slit, and FIG. 2 is cross-sectional plan view along line II--II of FIG. 1.
The ion source 22 of FIGS. 1 and 2 has a hollow cylindrical housing 40 made of a magnetoconductive material such as Armco steel (a type of a mild steel), which is used as a cathode. The cathode 40 has a cylindrical side wall 42, a closed flat bottom 44 and a flat top side 46 with a circular ion emitting slit 52.
A working gas supply hole 53 is formed in the flat bottom 44. The flat top side 46 functions as an accelerating electrode. Placed inside the interior of the hollow cylindrical housing 40 between the bottom 44 and the top side 46 is a magnetic system in the form of a cylindrical permanent magnet 66 with poles N and S of opposite polarity. An N-pole faces flat top side 46 and S-pole faces the bottom side 44 of the ion source. The purpose of a magnetic system with a closed magnetic circuit formed by parts the 66, 46, 42, and 44 is to induce a magnetic field in ion emitting slit 52. A circular annular-shaped anode 54 which is connected to a positive pole 56a of an electric power source 56 is arranged in the interior of housing 40 around magnet 66 and concentric thereto. The anode 54 is fixed inside housing 40 by means of a ring 48 made of a non-magnetic dielectric material such as ceramic. The anode 54 has a central opening 55 in which aforementioned permanent magnet 66 is installed with a gap between the outer surface of the magnet and the inner wall of opening 55. A negative pole 56b of electric power source is connected to the housing 40, which is grounded at GR.
Located above the housing 40 of the ion source of FIGS. 1 and 2 is a sealed vacuum chamber 57, which has an evacuation port 59 connected to a source of vacuum (not shown). An object OB to be treated is supported within the chamber 57 above the ion emitting slit 52. The object OB is electrically connected via a line 56c to the negative pole 56b of the power source 56. Since the interior of the housing 40 communicates with the interior of the vacuum chamber 57, all lines that electrically connect the power source 56 with the anode 54 and the object OB should pass into the interior of the housing 40 and the vacuum chamber 57 via conventional commercially-produced electrical feedthrough devices which allow electrical connections with parts and mechanisms of sealed chambers without violation of their sealing conditions. In FIG. 1, these feedthrough devices are shown schematically and designated by reference numerals 40a, 57a, 57b, and 57c.
The known ion source of the type shown in FIGS. 1 and 2 is intended for the formation of a unilaterally directed tubular ion beam. The source of FIGS. 1 and 2 forms a tubular ion beam IB emitted in the direction of arrow A and operates as follows.
The vacuum chamber 57 is evacuated, and a working gas is fed into the interior of the housing 40 of the ion source via a gas-supply tube 53a. An electric field is generated in the ion generation gap 58 and the ion-emitting slit 52 due to an electrical potential applied from the electric power supply 56 between the anode 54 and the upper cathode plate 46. As a result, a glow discharge occurs in the gap 58 after the potential reaches a predetermined value. A magnetic field is generated by the magnet 66 across the ion-emitting slit 52 whereby free electrons of the glow discharge begin to drift in a closed path within the crossed electrical and magnetic fields. When the working gas is passed through the ionization gap, the tubular ion beam IB, which is propagated in the axial direction of the ion source shown by an arrow A, is formed in the area of the ion-emitting slit 52 and in the accelerating gap between the anode 54 and the cathode 40.
The above description of the electron drift is simplified to ease understanding of the principle of the invention. In reality, the phenomenon of generation of ions in the ion source with a closed-loop drift of electrons in crossed electric and magnetic fields is of a more complicated nature and consists in the following.
When, at starting the ion source, a voltage between the anode 54 and cathode 40 reaches a predetermined level, a gas discharge occurs in the anode-cathode gap. As a result, the electrons, which, under of effect of concurrent electrical and magnetic fields, move along complex trajectories, are accumulated and held in the area of the ion-emitting slit 52 and in the anode-cathode gap 58. In fact, the aforementioned electrons drift along the closed-loop slit 52 and exist there over a long period of time. After being accelerated by the electric field, the ions generated in the anode-cathode gap due to collision of neutral molecules with electrons, pass through the ion-emitting slit 52 and are emitted from the ion source.
Strictly speaking, the aforementioned complex trajectories are closed cycloid trajectories. The phenomenon of drift of electrons in the crossed electric and magnetic fields is known as "magnetization" of electrons. The magnetized electrons remain drifting in a closed space between two parts of the cathode, i.e., between those facing parts of the cathode 40 which form the ion-emitting slit 52. The radius of the cycloids is, in fact, the so-called doubled Larmor radius R.sub.L which is represented by the following formula: EQU R.sub.L =m.sub.e V/.vertline.e.vertline.B,
where m.sub.e is a mass of the electron, B is the strength of the magnetic field inside the slit, V is a velocity of the electrons in the direction perpendicular to the direction of the magnetic field, and .vertline.e.vertline. is the charge of the electron.
A distinguishing feature of the ion source of the type shown FIGS. 1 through 3 is that efficient operation of the source is possible only when the source has the ion-emitting slit and the anode-cathode gap of predetermined geometrical dimensions. More specifically, the width of the ion-emitting slit 52 and the height of the gap 58 should be on the same order as the aforementioned Larmor radius.
When a working medium, such as argon which has neutral molecules, is injected into the slit, the molecules are ionized by the electrons present in this slit and are accelerated by the electric field. As a result, the thus formed ions are emitted from the slit towards the object. Since the spatial charge of electrons has high density, an ion beam of high density is formed. This beam can be converged or diverged by known technique for specific applications.
Thus, the electrons do not drift in a plane, but rather along cycloid trajectories across the ion-emitting slit 52. However, for the sake of convenience of description, here and hereinafter such expression as "electron drifting plane" or "drifting in the plane of ion-beam propagation" will be used.
The diameter of the tubular ion beam formed by means of such an ion source may reach 500 mm and more. The ion source of the type shown in FIG. 1 and FIG. 2 is not limited to a cylindrical configuration and may have an elliptical or an oval-shaped cross.
A disadvantage of the aforementioned ion source with a closed-loop ion-emitting slit is that the position of the tubular ion beam emitted from this source remains unchanged with respect to the surface of the object OB being treated. Furthermore, the aforementioned tubular beam has a non-uniform distribution of the ion beam current in the cross-section of the beam and hence on the surface of the object OB. More specifically, the ion current density across the tubular beam has two maximums in the areas corresponding to the closed-loop slit and one minimum in the center of the "tubular" profile of the beam.
With ever growing demands to the quality and performance characteristics of semiconductor devices, uniformity of treatment of semiconductor wafers becomes a critical issue. This is because even insignificant variations, e.g., in thickness of layers, causes significant variations in parameters of semiconductor devices. Therefore, at the present time deviations from uniformity within the range 5% (i.e., .+-.2.5%) becomes standard for such operations as etching, stripping (removal of resist), overcoating by sputtering metals or dielectrics, etc. The above ion-beam source with a closed-loop ion-emitting slit and electrons drifting in crossed electric and magnetic fields was given only as one specific example. Known in the art are ion-beam sources of many other types, e.g., the so-called end-Hall type ion-beam source described, e.g., by Kaufman H. R. et al. (Characteristics, Capabilities, and Applications of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria, Va.; Wykoff C. A. et al., 50-cm Linear Gridless Source, Eighth International Vacuum Web Coating Conference, Nov. 6-8, 1994) This ion source forms conical or belt-like ion beams in crossed electrical and magnetic fields. The device consists of a cathode, a hollow anode with a conical opening, a system for the supply of a working gas, a magnetic system, a source of electric supply, and a source of electrons with a hot cathode. Configuration of the electrodes used in the ion beam of such sources leads to a significant divergence of the ion beam. As a result, uniform distribution of current density across the beam can be achieved only in the center of the beam. Furthermore, the doze of irradiation with the ions will be essentially reduced simultaneously with a significant increase in the treatment time. This is because only a small portion of the beam which has uniformity is used. In other words, any single-beam ion source will have limits in their practical application for uniform treatment of large-area objects. Another disadvantage of the aforementioned ion-beam source is that the beam is perpendicular to the surface of the object. It is known, however, that when the ion beam fells onto the object at an angle different from normal, the efficiency of sputtering can be increased by three or more times.
In general, methods for providing uniformity in distribution of ion-beam current density across the beam and on the surface of objects treated with ion-beam sources (hereinafter referred to simply as "uniformity") can be roughly divided into the following categories:
1) Uniformity achieved due to beam divergence; PA1 2) Uniformity achieved due to multiple-cell structure of the ion source; PA1 3) Uniformity achieved by alternatingly or periodically changing position of ion beam with respect to the object.
An example of an ion source in which uniformity is achieved due to beam divergence is a device disclosed in U.S. Pat. No. 6,130,507 issued Oct. 10, 2000. This patent application discloses a closed-loop slit cold-cathode ion source where uniformity of treatment of an object is achieved by shifting the anode with respect to the cathode or vice verse.
Such displacements cause variations in relative positions between the object and the beam whereby even with some non-uniformity in the ion current density distribution in the beam, the surface of the object is treated with an improved uniformity.
A disadvantage of such a device is that the ion source or the ion-beam sputtering system should have movable parts which makes the construction of such source or system more complicated and expensive.
An example of a device in which uniformity achieved due to multiple-cell structure is an ion-beam apparatus described in USSR Inventor's Certificate No. 865043. As shown in FIG. 3, which is an elevational sectional view of the ion-beam source 100 of the aforementioned type, the device is made in the form of a multiple-cell source having two cathode plates 102 and 104 which function as magnetic poles. An anode plate 106 with openings is placed between cathode plates 102 and 104. Cathode plate 104 has rods 108a, 108b, 108c, which extend from cathode plate 104 to second cathode plate 102. Second cathode plate 102 has openings 110a, 110b, 110c coaxial with respective rods 108a, 108b, 108c and with openings in anode plate 106. The anode-cathode assembly is supported by a cup-shaped housing 112 of a nonconductive material, such as a ceramic, which contains an electromagnetic coil 114 for generating the aforementioned magnetic field in a anode-cathode space of ion source 100. In a conventional manner, entire ion source 100 is placed into a sealed vacuum chamber 118. A working medium is supplied to vacuum chamber via a working medium supply channel 116.
Thus, each opening 110a, 110b, 110c in cathode plate 102 and a respective coaxial rod 108a, 108b, 108c of the device form an individual ion-beam source of the type described above, i.e., of the type disclosed in Russian Patent No. 2,030,807. More specifically, the end of each rod and the adjacent opening in cathode plate 102 form a closed-loop ion-beam emitting slit, so that all rods and the openings in the second cathode plate form a plurality of such slits. In the context of the present invention, a combination of one rod with a respective opening will be referred to as a "cell", and the ion-beam source of this type will be called a "multiple-cell type ion-beam source". Cathode plates 102 and 104 are electrically isolated from anode plate 106 and can be grounded or connected to a source of a negative potential via a conductor 122, and anode plate 106 is connected to a source of a positive potential (not shown) via a conductor 124.
In operation, a working medium is supplied through channel 116 to the accelerating and ion-generating space between anode plate 106 and cathode plates 102, 104, and a potential difference is developed between the cathode plates and the anode plate. This generates crossed electric and magnetic fields in the anode-cathode space. These fields hold drifting electrons which ionize the working medium and compensate for the spatial charge of the ion beams IB1, IB2, IB3, which are emitted toward an object OB1 via openings in the second cathode plate. Object OB1 is fixed inside vacuum chamber 118.
Although the multiple-channel ion-beam source of the type described above to some extent improves uniformity of ion-current density distribution on the surface of an object being treated, adjustment of distribution of the beam current density on the surface of the treated object OB1 is impossible.
This problem is partially solved in the apparatus 100' shown in a partial side sectional view in FIG. 4 and described in pending U.S. patent application Ser. No. 240,469 filed by the same applicants on Jan. 30, 1999.
As shown in FIG. 1, each rod 108a', 108b', . . . has means for individually adjusting magnetic fields in individual ion-emitting slits 110a', 110b', . . . . This, in turn, allows for individually adjusting conditions for ionizing electrons in the ion-emitting slits and, hence, the density of ions emitted through individual ion-emitting slits.
However, the ion-beam source of FIG. 4 produces a plurality of ion beams all of which have direction essentially normal to the surface being treated. In other words, the efficiency of sputtering in this source is still much lower than in the case of angle of incidence different from 90.degree.. Furthermore, the provision of individual electromagnetic coils makes the construction of the ion-beam source more complicated and expensive.
An example of an ion-beam apparatus in which uniformity is achieved by alternatingly changing positions of the ion beam with respect to the object is a device described in U.S. Patent No. 6,037,717 issued on Mar. 14, 2000. This application describes a cold-cathode ion source with a closed-loop ion-emitting slit which is provided with means for generating a cyclically-variable, e.g., alternating or pulsating electric field in an anode-cathode space. These means may be made in the form of an alternating-voltage generator which generates alternating voltage on one of the cathode parts that form the ion-emitting slit, whereas the other slit-forming part is grounded. The alternating voltage deviates the ion beam in the slit with the same frequency as the frequency of the alternating voltage. The cold-cathode ion source may be of any type, i.e., with the ion beam emitted in the direction perpendicular to the direction of drift of electrons in the ion-emitting slit or with the direction of emission of the beam which coincides with the direction of electron drift.
Displacements of the beam cause variations in relative positions between the object and the beam whereby even with some non-uniformity in the ion current density distribution in the beam, the surface of the object is treated with an improved uniformity.
A disadvantage of all ion-beam sources described above is that none of them allow evacuation of gases from the vacuum chamber through the central opening of the ion beam source. Such a demand, however, may occur in some applications.
U.S. patent application Ser. No. 240,468 filed by the same applicants on Jan. 30, 1999 describes a combined ion-source and sputtering magnetron apparatus having a coldcathode ion source which emits the ion beam in the radial inward or outward direction onto the surface of the magnetron target at an oblique angle to the target surface. This increases efficiency of sputtering. Furthermore, the ion source has a ring-shaped configuration so that gases can be evacuated from the vacuum chamber through the central opening of the ion beam source.
Although such a single-beam ion source produces a converging ion beam with self-overlapping portions, which to some extent can improve uniformity of treatment, the uniform surface area is limited and uniformity is low and cannot be adjusted.